1. Field of the Invention
The present invention relates to laser diodes and more specifically, it relates to a low cost laser diode array.
2. Description of Related Art
Laser diode arrays are used in a wide range of commercial, medical and military applications: materials processing (soldering, cutting, metal hardening), display technology/graphics, medical imaging (MRI) and surgical procedures (corneal shaping, tissue fusion, dermatology, photodynamic therapy), satellite communication, remote sensing, and laser isotope separation. In certain solid-state laser applications it is desirable to use laser diode arrays to optically excite, i.e., "pump," the crystal hosts. Diodes offer a narrow band of emission (reducing thermal lensing), compactness, high electrical efficiency and higher reliability as compared to flash lamps. Despite these numerous advantages, however, diode-pumped solid-state lasers (DSPSLs) have gained slow market acceptance due to the high cost associated with the laser diode array pumps. Significant diode array cost reductions would enable wide deployment of DPSSLs and new architectures to be realized that were previously cost prohibitive. In particular, low-cost diode arrays would bolster the inertial confinement fusion (ICF) and inertial fusion energy (IFE) programs that require low-repetition rate laser diode arrays in very high volumes.
Historically, much of the research and development in this area was devoted to solving diode material and fabrication issues in order to improve the yield and reliability of laser diodes. High quality InAlGaAs and InGaAsP laser diodes are now commercially available for pumping Nd:YAG at .about.810 nm. As much as 100 W/cm of peak power is possible under pulsed operation, and over 10,000 hours of continuous operation (CW) in commercial systems has been demonstrated at reduced power levels (20-30 W CW). Although these types of performance improvements have led to cost reductions in the past, there has not been a complementary improvement in the packaging technology, which is now limiting further cost reductions from being achieved.
To date, most packaging/heatsink schemes use a "rack and stack" architecture. In this method, individual laser bars are fabricated into sub-assemblies, and the sub-assemblies are then bonded together to produce larger two-dimensional arrays. Labor intensive steps associated with handling individual components prevents the production of arrays in large volume and in high yield. To alleviate this problem, a "monolithic" fabrication approach known as "bars-in-grooves" was proposed. This process was comercialized by Laser Diode Array Inc. and it represents the only "monolithic" technology that is commercially available today. There are trade offs associated with using a monolithic technique (e.g. by LDA Inc.) and the salient issues are discussed below.
The grooves must be deliberately over-sized to facilitate mounting the bars (as well as to allow for a range of diode bar thicknesses). Accurate final placement of the laser bar is therefore difficult to achieve as solder is used to fill in the void left by the over-sized grooves. This prohibits accurate collimation (tensing) of the laser diodes which is desirable in "high-brightness" applications that are often used in "end-pumped" geometries. More importantly, flowing solder around the bars can damage, or short-out bars which lowers yield and represents a serious liability to packaging costs of a completed array. Either that, or the strict process controls and/or lower yield of "suitable" bars that is necessary poses a cost penalty with this soldering technique. The following invention improves upon the limitations of the former "bars-in-grooves" method, while still benefiting from being a monolithic or quasi-monolithic approach. The placement of the laser diodes is well defined, and the soldering process can be extremely well controlled, or not used at all, which ensures a high yield that is crucial for a low-cost high yield production of laser diode arrays. It is emphasized that in the description by Karpinski et al. (U.S. Pat. No. 5,040,187), there is a method for flexing the substrate in order to facilitate loading laser bars. However, both the groove width and diode bar thickness would have to be controlled to such a high level of accuracy that this approach appears to be impractical. The present invention allows a practical implementation of a "solderless" contact because individual springs can accomodate any thickness variations of the individual components.